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This loop of GOES-8 split window IR imagery shows the spread of the ash plume following the eruption of the Popocatepetl volcano near Mexico City at 19:02 UTC on 04 December 1998. This split window IR product is created by taking the difference in brightness temperatures between the 10.7 micron IR and the 12.0 micron IR -- differences in the emissivity of silicate particles in the ash cloud lead to negative values in the split window IR product. The volacnic ash plume (blue, yellow and red enhancement) can be seen spreading mainly to the north and northwest (toward Mexico City), with part of the plume spreading to the south.